فصلنامه دریا فنون
سال هفتم شماره 1 (پیاپی 19، بهار 1399)

Either transferring of real-time under water medicine signals or the ability of Linear Predictive Coding (LPC) compression level of signals are important, so in this paper real-time sending of vital signs of divers with the LPC compression is presented. On the other hand, it must be mentioned that the life of diver is related to accurate sending of marks and the extreme similarity of original signal with the compressed signal, so accuracy of compression in this area is so important. Despite of high capability in compression in LPC, this method is not suitable for this application. In this paper, to increase accuracy of compression and decreasing of forecast error of LPC coefficients, the optimized algorithm of Humpback whale is used. In addition, the designed algorithm is practically tested on one specific sonobouy. The result is additionally compared with standard LPC and both compression method based on discrete cosine transform and sampling low rates which obtained results show that proposed algorithm has an increasing accuracy.

Multilevel inverters have great applications in medium-voltage and high-power applications and also have found successful industrial applications. Modified full bridge inverter is one of the most popular topologies among the multilevel inverters because of lower number of switches and simpler structure than others. In this paper, a seven-switch and seven-level modified full bridge topology is proposed. Compared to the practical seven-level inverters like neutral point inverters, the proposed inverter reduces three active switches and has lower conduction loss and simple control. The input voltage to the given inverter is a DC source (Vdc) which has been shared between three identical capacitors and it is capable of producing output of seven level (Vdc, 2Vdc/3, Vdc/3, 0, -Vdc, -Vdc/3, -2Vdc/3). As more steps are added to waveform, the harmonic distortion of the output waveform decreases. The proposed modulation enables the proposed inverter to operate under both active and reactive power conditions. A 1KVA single-phase sample circuit is simulated by the PSPICE software to verify the validity of the proposed topology and modulation method. The results show the proposed topology is suitable for photovoltaic (PV) applications.

The increasing in energy demand by the limitation of fossil-fuel resources and the environmental pollution caused by fossil fuels has attracted much attention to renewable energy sources. In the ocean thermal energy conversion system , the temperature difference between the warm water of the sea surface and its deep cold water is used to generate electric power. Due to low temperature difference between hot and cold sources, thermal efficiency of these systems is lower compared to fossil-fuel power plants. In the present study, as a proposed cycle to increase the production capacity and thermal efficiency of the cycle, the warm water output from the thermal power plant condenser was used instead of surface water, and the two-stage turbine and reheating between the two stages were used. The energy and exergy analysis of proposed cycle was done. The results showed that average of electricity produced in cycle per month increases 27.52 MW and energy and exergy efficiencies were 3.34% and 17.2%, respectively. In the following, proposed cycle was run using warm water output from Neka combined cycle power plant condenser. The results showed that by using the outlet water from the condenser of combined cycle power plant the average net electrical power was produced by 17.72 MW per month. Also, an average of 18829 tons per day of fresh water was produced, and the thermal efficiency of the combined cycle improved by 1.87%.

Trimaran is a multi-hull ship that consisting of a main slender hull and two smaller outriggers. These outriggers are connected by a transverse cross deck to the main hull. The position and geometry form of the outriggers have a significant effect on the stability, dynamic motions and wave loads on this modern ship. In this paper, the effect of the outriggers geometry on heave and pitch motions of a trimaran ship, considering the inboard and outboard asymmetric geometry hull form are investigated. A model of ship has been tested in a towing tank at different speeds and different frequencies in forward sea. The results showed that due to the proper interaction between the main hull and the outriggers in terms of using the side body with the outboard asymmetric form, the responses are less than those of the inboard asymmetric hull. Also, there are local maximum and minimum points on ship responses in zero speed conditions due to the heave and pitch coupling in their respective frequencies.

Waterjet Propulsion System theory is a combination of pump and propeller. The pressure in the leading edge and suction side of a propeller at high rotational speed will be less than the saturation pressure of liquid; therefore cavitation occurs. In this paper, using the computational fluids dynamics, torque, efficiency and head parameters are predicted in non-cavitation conditions in steady and unsteady state. The numerical results compared with experimental data. In cavitation condition, the flow is modeled in two phases. The pattern of cavitation formation on the rotor was also studied. Furthermore for another flow coefficient, thrust breakdown value is obtained. A parametric study was also performed to investigate the role of the stator in neutralizing the torque of rotor. The structured mesh and MRF were used in the finite volume numerical simulation.

The effort has been made in this research to reduce the vibrations of the jacket platforms vibrated by irregular waves through dynamic vibration absorbers (DVAs). The production of intelligent materials and their application in the design of control devices have made significant progress in the past two decades. The shape memory alloy (SMA) is an intelligent material highly capable of damping vibrations. Hence, use has been made in this study, of the shape memory alloy tuned mass damper (SMA-TMD) to control the vibrations of the offshore jacket platforms. Here, the equations governing the platform/damper vibrations have been developed by simulating them as a multi-degree of freedom lumped mass system and analyzed by the direct integration method. To evaluate the structural damping force and the SMA hysteretic behavior, the extended Rayleigh damping model and idealized multi-linear constitutive model have been used, respectively. As a case study, a 90 m-high jacket platform (in 80 m-deep water) has been equipped with SMA-TMD and its dynamic responses have been estimated. An average decrease of 16 percent in the root mean square of the deck displacements and 75 percent reduction in the power spectral density function (in the resonance state) are the results of jacket platform equipped with optimized SMA-TMD.

The damage mechanism of armor layer is a very complicated phenomena and many uncertainties affect the exact estimation of different effective parameters in armor design. In the present study, the stability of armor layer was investigated by using a reliability-based framework. Monte Carlo (MC) simulation technique and First Order Reliability Method (FORM) were established to determine the stability of armor layer placed on Noshahr port breakwater as a case study. Results showed that due to existing uncertainties, the stability of designed armor weight which was calculated from deterministic method was low and further considerations are needed to decrease the damages. In addition, a linear relationship between safety factor and reliability index was determined for the case study. Results showed that by increasing the reliability index parameter, the safety factor should be increased. In addition, the corresponding saftety factor for the target reliability index of 3 was determined as 3.6. Next, sensitivity analysis was performed to examine the impact of each random variable on the probability of armor stability in the reliability method. Results showed that the most critical parameters affecting the reliability of armor weight were wave height and dimensionless damage level. These parameters need to determine more accurately to decrease the failure of armor layer efficiently. Finally, the effect of parameters’ uncertainties on reliability analysis of armor layer was investigated. Results showed that for all of the effective parameters by increasing the parameters’ uncertainties, the stability of armor layer decreases. Hence, by increasing the coefficient of variation of significant wave height from 0.05 to 0.5, the reliability index decreaded for about 70%.

In this paper, the application of Endurance Time (ET) method in seismic analysis of pile-supported wharves is investigated and compared with the other nonlinear seismic analyses. For this purpose, Emam-khomeini port is considered. This wharf, which has a long fundamental natural period and is classified as a flexible structure is modeled in three different states, initial, damaged and retrofitted states. Subsequently, each state is analyzed by Capacity Spectrum Method (CSM), Time History Analysis, and ET method. Comparing to time history analysis, the average amount of structural responses in ET method increase by 17, -8, and 7 percent in initial, damaged, and retrofitted states, respectively, and these amounts noticeably increase by 53, 49, and 47 percent when the responses are compared with CSM method. In addition, in comparison with time history analysis, in ET method, the dispersion of responses rises by 60 percent in the initial state, 19 percent in the damaged state, and 34 percent in the retrofitted state. These amounts are 47, 66, and 17 percent comparing to CSM method. Finally, it can be inferred that ET method is an appropriate alternative to nonlinear time history analysis since the same results are mostly obtained by selecting fewer number of records and performing fewer number of runs.

In this study, a wave-energy converter based on the Oscillating Water Column is simulated using numerical method. Reynolds averaged Navier-Stokes equations are solved by applying a numerical method based on computational fluid dynamics and using an open source OpenFOAM code. A suitable solver for considering the effects of free surface as well as the effects of turbulence is chosen. Numerical validation was performed based on the published experimental results and for a 2D problem. The changes in the water free surface inside the converter, the air pressure as well as the converter efficiency were presented and the effects of the wave parameters on the hydrodynamic performance of the converter were evaluated. Also, the forces acting on the OWC in different conditions are presenterd and can be used in the design of coastal structures. Using the current method and generating regular waves in a numerical tank, and accurately simulating the hydrodynamic characteristics of the converter, and examining the exact parameters of fluid that is not measurable in the experimental analysis, provides a precise and appropriate solution for achieving the best dimensions and location of the converter in terms of the real sea conditions. The accurate results of numerical simulation against the available experimental data and the possibility of detail analysis of flow characteristics versus experimental measurements are highlights of this study.

In this paper, robust adaptive fuzzy sliding mode control is designed to control the Permanent Magnet (PM) stepper motor in the presence of model uncertainties and disturbances. In doing so, the nonlinear model is converted to canonical form, then, for designing the controller, the robust sliding mode control is designed to decrease the effects of uncertainties and disturbances. A class of fuzzy systems to approximate the ideal input is designed. Stability of closed loop control is also guaranteed by the Lyapunov function employed in controller design. Compared to previous studies that have used position, velocity, and currents phases in the feedback loop to produce and track control signal, the proposed algorithm only needs to measure position and velocity of the shaft for controlling performance of the motor. All the other parameters of motor and load torque are assumed to be unknown. The simulation results show that the control strategy is effective to the motor position tracking error and robust to overcome uncertainties and disturbances..